It is well known that in some eyewear devices the visor included therein generally has a cylindrical shape.
One way to obtain such a shape, especially used in case of the most common thermoplastic optical materials such as polycarbonate, polymethylmethacrylate or polyamides, is to manufacture the visor by injection molding in an optically compensated mold.
A visor obtained in this way is described for example in International patent application WO 97/03579.
Although this technique may allow to manufacture a cylindrically shaped visor having the required optical characteristics, the investment costs to set up the necessary injection molding equipment are so high that they are generally justified only in case of mass-production of a large number of pieces, i.e. can be afforded only by large enterprises.
In case of smaller productions or whenever it is not economically affordable the use of such an expensive technique, sheet thermoforming still remains the only feasible method of manufacturing visors.
Nowadays, this technique is commonly carried out by thermoforming an optical sheet made of plastics material having two parallel surfaces, i.e. by thermoforming a plano-parallel optical sheet. The visors thus obtained are used for example in sport goggles, helmets and in certain models of fashion sunglasses.
More precisely and as best shown in FIGS. 1 and 2, a cylindrically shaped visor 4 is obtained from an optical sheet 1 having two parallel inner and outer surfaces 2, 3 by cutting the optical sheet 1 in shape, heating it at the softening temperature, bending it with the right curvature, cooling it down to a room temperature so as to obtain a visor 4 which is then mounted or fastened, for example, on the frame on the helmet temples.
However, the cylindrically shaped visor obtained in this way is not optically correct since the bending operation of the optical sheet 1 induces a negative optical power that can be easily measured with a telescope commonly present in any optical lab.
The reason for the negative optical power is that the light rays (shown by the arrows in FIG. 2) diverge when they pass through the two cylindrical concentric surfaces 2, 3. This negative power increases as the thickness of the optical sheet 1 increases and/or when the refractive index of the material constituting the same is increased and/or when the curvature of the final visor to be obtained is increased.
For example, if a 1.3 mm thick plano-parallel optical sheet 1 made of diethylenglycol-bis-allyl-carbonate (better known under the trade name of CR39®) with a refractive index of 1.523 is bent with a curvature radius of 87 mm (base 6), a negative spherical aberration of about −0.08 diopter (D) results.
If the thickness is increased to about 2 mm and/or the refractive index is higher than 1.523, the negative power becomes higher and the visor obtained will easily exceed the maximum value allowed by the international standards for sunglass lenses (for example −0.09D for Optical Class 1 defined by the European Standard EN 1836).
Accordingly, the cheaper thermoforming technique has been used—to date—only to manufacture cylindrically shaped non optically correct visors having a maximum thickness of about 2 mm.